RE: Prop choices for .020, .049 and .061
Meowy84, in Ptulmer's example the RPM's are the same because the loading is the same. Lower pitch will load the engine less. The rule of thumb is 1 pitch is roughly equivalent to 1" of diameter. So a 10X7 and a 12X4 give about the same loading and about the same RPMs. A heavier model doesn't necessarily need more thrust unless you are looking for better vertical. Heavier models often require more airspeed to fly than a lighter model. A good example is in some geared electric models where the quest for thrust has become so extreme that they hardly fly on the wing anymore, and have to be built extremely light so that they can fly on the prop. Your best bet is to try a range of props with the knowledge that if, after a test flight, you want more thrust you need to go up in diameter and down in pitch and vice versa for speed.
A PeeWee .020 comes with a 4.5X2 prop, but can spin up to a 5X3. A Norvel .061 can use anything from a 5X3 to 6X4. I prefer an APC 6X2 for the .061, but a COX 5X3 or an APC 5.7X3 work well too.